Built-in antimicrobial plastic resins and methods for making the same

ABSTRACT

A built-in and process-adaptive formulation of antimicrobial commodity thermoplastic resins with mixed compositions comprising of a polymer, a backbone linker, and a non-labile antifouling and biocompatible coupling agent which is melt-processable and enabled to be manufactured into finished products in the form of solid, monolith, tube, composite, fiber, film, sheet and varnish without the prerequisite of biocides or antimicrobial additives is disclosed. The said formulation is adapted to thermoforming and thermal curing processes including but not limited to melt compounding, spinning, extrusion, molding, compression foaming and drawing. The antimicrobial property is attributed to the persistent formation of a non-stick bacteria-repellent tethered layer in which the antifouling component of the said formulation is heterogeneously phase separated and/or surface migrated to the surface after product forming in order to minimize adsorption and/or colonization of bacteria.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part patent application of U.S.non-provisional patent application Ser. No. 15/318,585 filed on Dec. 13,2016, which is a national phase patent application of PCT applicationnumber PCT/US2016/070500 filed on Jan. 8, 2016 claiming benefit fromU.S. provisional patent application Ser. No. 62/124,973 filed on Jan. 9,2015, and the disclosure of which are incorporated herein by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates to antimicrobial plastic resins and themethods for making the same, in particular, relates to built-in,biocide-free and safe-to-use antimicrobial plastic resins and themethods for making the same.

BACKGROUND

Despite the biocidal actions to kill the bacteria after attachment tothe substrate through several mechanisms, such as inhibition of thesynthesis of the cell wall, the nucleic acid, the protein and finallythe distortion of the cellular metabolism (Murray et al. MedicalMicrobiology, 4^(th) ed.; Mosby: St Louis, 2002), there are other morenatural microbial prevention strategies that can be applied onto thepolymer surfaces. While there might be others, one very good example ofdemonstration is the inherent antimicrobial property of chitosan, anatural polysaccharide derived from the shrimp (Chung et al. ActaPharmacol. Sin. 2004, 25, 932). The most common antimicrobial productsadopt bacteria killing as the strategy through biocidal actions andtherefore the bacterial prevention effectiveness heavily depends on therelease and replenishment of the leachable biocides that are required tomigrate to the surfaces from the inner matrix. The biocides mustdissipate off the surface for killing a microbe, leaving behind lessantimicrobial effectiveness for future microbial encounters. This meansthat the microbial protection is perishable and has a fixed shelf life.Furthermore, over extended time, the adsorbed microbes on the substratewill gradually adapt to the surfaces, if the biocidal action is notimmediate or if biocides are underdosed at levels below the minimuminhibitory (MIC) or minimum bactericidal concentrations (MBC). Eachbacterial isolate has its specific MIC or MBC. This aids to selectbiocide-resistant microbes.

WO2005021626 discloses a built-in antimicrobial formulation for acrylicpolymers by mixing with several organic antimicrobial additives inarticles.

US20140008324 discloses methods for processing plastic substrates,comprising at least one of injection molding, thermoforming, orextruding, having inorganic antimicrobial microparticles within and thenusing plasma etching which results in the removal of a portion of thesubstrate surface and thereby exposing material within the substrate.

Photocatalysts, such as ZnO and TiO₂, exhibit disinfecting effectsagainst gram-positive and gram-negative bacteria and they work onlyunder UV irradiation (Dhanalakshmi et al. Mater. Express 2013, 3, 291).

Others employ electrostatic method by the use of cationic polymers, suchas chitosan, a natural polysaccharide. The bactericidal action ofchitosan targets at the cell membrane of the bacteria which isnegatively charged. The bacteria preferentially adsorb to the polymersurface and the strong adhesion leads to a gradual increase in theircell permeability and eventually intracellular dissolution due to thedistortion of the charge distribution of the membrane (Chang et al. J.Agric. Food Chem. 2012, 60, 1837). However, this approach is notuniversal to target at a broad spectrum of bacteria bearing differentcell membrane charges; some bacteria can be positively charged.

On the other hand, the lower-molecular weight polymers of chitosan werefound to be able to diffuse and permeate through the porous membraneinto the cells and form stable complexes with DNA. This subsequentlyprevents the DNA transcription activities, thus leading to inhibition ofthe proliferation and even the death of bacteria (Kenawy et al.Biomacromolecules 2007, 8, 1359).

Another biocidal approach requires complex nanoscale features andpattern topographies, such as nanopillars, which can be found on theinsect wings (Pogodin et al. Biophys. J. 2013, 104, 835).

DE19535729 discloses a biocide-free coating free from biocides based onorganofunctional silanes and fluoroorganosilanes and/or theirhydrolysates and/or condensation products but the invention is limitedto coatings which function properly with metal substrates, such asaluminum foil.

All these cater for the development of the new approach via abacteria-repellent layer to be permanently and stably formed on thesurface of a commodity plastic article and to prevent adhesion andaccumulation of bacteria. This overcomes all the drawbacks that comewith the conventional surface coating and/or combined with biocidalapproaches.

One feasible antifouling structure is based on electrostatics viacharge-bearing polymers, polyelectrolytes, polysaccharides andpolypeptides containing amino, quaternized, carboxylated, sulfonated,phosphate, boronate entities and other metal oxides, complexes and theirderivatives, of which the zeta potentials can be finely tuned by pH,counterion and charge valence.

U.S. Pat. No. 8,545,862 discloses an anionic/cationic polyelectrolytecomplex, for example, a composition consisting essentially of aderivative or copolymer of poly(acrylic acid) or polystyrene sulfonate,to impart antimicrobial properties to an article.

WO2012065610 discloses a long-lasting antimicrobial coating for fabricscomprising a polymeric quaternary ammonium (quat).

EP2627202 discloses an antimicrobial peptide comprising Brad or anactive variant thereof as a food preservative to prevent or inhibitspoilage of a foodstuff by a microorganism.

Just this approach, however, is not versatile enough. It is because somebacteria, plasma proteins and red blood cells carry negative charges ontheir cell membranes and they are expected to show very similarelectrostatic repellent behavior against a substrate surface of the samecharge. While other species of bacteria can be positively charged, suchas Stenotrophomonas maltophilia, this approach is therefore notuniversal to target at a broad spectrum of bacteria as differentiated bythe cell membrane charges.

The second feasible structure of antifouling groups is derived fromneutral polymers, such as poly(2-hydroxyethyl methacrylate) (polyHEMA),poly(ethylene glycol) (PEG) and Zwitterionic polymers and aheterogeneous polymer system of mixed charges comprising of cationic andanionic functionalities onto the plastic surface (Sin et al. Polym. J.2014, 46, 436). PolyHEMA shows the repellent properties because of itsstrong hydrophilicity so that it can displace the deposition of bacteriaby a tightly bound hydration layer. Hydrophilic surfaces are apparentlyhelpful to avoid bacterial adhesion. PEG however uses the stericexclusion effect to resist the protein and platelet adsorption. Previousdata also suggested that the adherence of the bacteria was determined bythe composition and the chemical nature of the pre-adsorbed proteinlayers coupled with the surface hydrophilicity. Zwitterionic polymersare bioinspired from the Zwitterionic phospholipid structures of thecell membranes which are well-known to be bio-inert. As different fromthe hydrophilic polymers, the betaine-based Zwitterionic polymers canfinely tune the electrostatic interactions with the nearby watermolecules and control the non-specific protein adsorption. Siedenbiedelet al. reveal successful examples of applications of Zwitterionicpolymers for prevention of bacterial adhesion to the surfaces afterchemical modification (Siedenbiedel et al. Polymers. 2012, 4, 46).

In the third feasible approach, the antifouling property of the plasticmaterial can be achieved by modifying the chemical group functionalitywhich in turn changes the surface hydrophobility via end terminationand/or grafting of a polymer chain with alkyl, hydroxyl, fluoroalkyland/or aromatic entities (Nie et al. J. Mater. Chem. B 2014, 2, 4911),completely different adhesion and physicochemical behaviors ofbiomolecules onto the surfaces will be acquired.

SUMMARY OF THE INVENTION

Conventional methods of fabrication of antifouling andbacteria-repellent surfaces are achieved mostly by surface chemicalgrafting, vacuum deposition, in-situ polymerization, spin-coating anddip-coating of related antifouling materials which have low throughputs.Another technical problem with surface modification is the prerequisiteof post-treatment, such as thermal curing, photoreactions, etc. in orderto ensure a uniform coverage of the coating on the articles whereas theshape limitations with curved, inner lumen and fine features are oftenencountered for different finished consumer products. While commodityplastic products are mainly manufactured by thermoforming processes,injection molding and extrusion in a large-scale and continuous mode ofoperation in plastic industries, any add-on surface modification and/orpost-treatment processes to the existing production lines are certainlynot desirable to manufacturers in consideration of the new capitalinvestment.

The objective of this invention is to develop biocide-free resinsovercoming the shortcomings of limitation to surface coatings orleaching out in existing technologies. Neither bactericidal norbacteriostatic, the biocide-free resins' surface shows built-in bacteriarepellent performance rather than killing performance after productforming.

The present invention is related to a reformulated thermoplastic resinwith built-in bacteria-repellency and adaptability to thermoformingprocesses. The nonfouling functionalization of a base polymer isperformed by reactive melt extrusion of a commercial thermoplasticresin, such as maleic anhydride bearing polyethylene, with anappropriate antifouling coupling agent comprising of one or more similarbio-repellent structures in the said chemical approaches and/or otherfree-radical initiators and acid/base catalysts, such as sodiumperiodate, azobisisobutyronitrile, benzoyl peroxide, dicumyl peroxide,potassium persulfate, p-toluenesulfonic acid, 4-dimethylaminopyridine,stannous chloride, dibutyltin dilaurate and trimethylsilyl chloride,thus leading to a masterbatch resin after cooling and pelletization. Theresin is then molded into finished articles which display antimicrobialperformance after a thermoforming process. The incorporated antifoulinggroups of the invention can phase segregate and inhabit the surfacesduring article forming processes, hence introducing permanent barriersagainst bacterial adhesion via a chemically-stable repellent surface onthe neat plastic article. The thermoplastic resin includes but notlimited to the family of polyolefin, polyether, polyvinyl, polyester,polyacetal, polyamide, polyurethane, polyacrylate, polycarbonate,polyimide, polyphthalate, polysulfone, polythioether, polyketone,epoxide and other elastomeric polymers, such as silicone, polyisoprene,acrylonitrile butadiene styrene and ethylene vinyl acetate. Because ofthe low-cost and high-volume production capability using thermoformingprocesses, typically melt extrusion and injection molding, theformulation is melt-processable and exhibits strong thermal chemicalstability up to a temperature as high as 350° C. Moreover, it tends notto interfere with the manufacturer's production line and the physicalbulk properties, such as optical transparency, thermal conductivity,mechanical stiffness, electrical conductivity, dielectric strength andflammability rating, of the formed articles comprising of theformulation. The articles can be in the form of composite, fiber, sheetand varnish.

After thermoforming, a non-stick bacteria-repellent layer of polymerbrush on the article surface is formed by heterogeneous phaseseparation, crystalline ordering and/or surface-directed migration ofthe backbone-grafted antifouling groups of the present invention tominimize the initial physical or chemical adsorption of motileplanktonic bacterial cells and later colonization of the harbored cellsleading to formation of irreversible biofilms that can withstandhost-defense measures or any antibiotics in use. The tethered layer hascovalently been tied with the neat matrix after article forming. Thisprovides unperishable protection of the surface from bacterialattachment. The said brush layer is not bactericidal or bacteriostatic.This means that the mechanism of action for microbial prevention is notachieved by killing or growth inhibition of bacteria. No leachable andnon-biodegradable biocidal additives are involved to move to the surfaceto work. This can entirely eliminate the chance to generatebiocide-resistant bacteria. The invention is therefore safe-to-use inthe way that the antifouling layer is biocompatible, non-cytotoxic anddoes not lead to either skin allergies on contact or persistentbioaccumulation in eco-system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the preparation process of anembodiment of the present invention.

FIG. 2 is an ATR-FTIR spectrum of PEG-bearing styrene-maleic anhydridecopolymer (SMA-PEG) and styrene-maleic anhydride copolymer (SMA).

FIGS. 3A and 3B are TGA and DSC thermograms, respectively, of anantimicrobial PP resin (PP/SMA-PEG) using SMA-PEG as masterbatch.

FIGS. 4A and 4B are TGA and DSC thermograms, respectively, of anantimicrobial PC resin (PC/SMA-PEG) using SMA-PEG as masterbatch.

FIG. 5 is an ATR-FTIR spectrum of PEG-bearing maleated polypropylene(PP-MA-PEG) and maleated polypropylene (PP-MA).

FIGS. 6A and 6B are TGA and DSC thermograms, respectively, of anantimicrobial PP resin (PP/PP-MA-PEG) using PP-MA-PEG as masterbatch.

FIG. 7 is an ATR-FTIR spectrum of PEG-modified maleated olefin bearingpolypropylene (PP/PP-MA-C/PEG) and maleated olefin bearing polypropylene(PP-MA-C).

FIG. 8 is an ATR-FTIR spectrum of N-(4-hydroxyphenyl)maleimide-modifiedPEG bearing polypropylene (PP/HPM-PEG).

FIG. 9 is a representative plate count result (100× dilution) ofEscherichia coli adsorption on specimens thermoformed from antimicrobialPP resin.

FIG. 10 is a representative plate count result (100× dilution) ofEscherichia coli adsorption on specimens thermoformed from antimicrobialPC resin.

FIG. 11 is a representative Escherichia coli adsorption test result onspecimens injection-molded from antimicrobial PE resin.

FIG. 12 is a graphical representation of viability of HaCat cells onfilm specimens prepared by hot pressing of different chemicals: “Medium”refers to the film specimen with the medium only as a control; (1) and(2) refer to the film specimens with pristine PP and an antimicrobial PPresin (SMA/SMA/PEG/CA), respectively.

FIG. 13 is a flow chart showing the minimum essential medium (MEM)elution methodology described by the ISO 10993-5 standard.

DETAILED DESCRIPTION OF THE INVENTION

To illustrate the structure and advantages of the present invention,below is the detailed description of the present invention incombination with the figures and embodiments.

The present invention discloses an antimicrobial thermoplastic resin,which comprises a masterbatch and a basic plastic, the masterbatch isprepared by grafting an antifouling reagent onto an intermediate. Theintermediate is prepared by grafting a reactive linker onto a basepolymer backbone.

The base polymer backbone is a synthetic vinyl polymer with R groups.The R groups are linear and/or multi-armed chemical structures withhomo- or hetero-substituted alkyl, alkenyl, alkynl, aryl, acyl, alkoxyl,thionyl, cyano, azo, silyl groups, halogens and/or cyclics. Theantifouling reagent is a hydrogel forming polymer, constituting polyol,polyoxyether, polyamine, polycarboxylate, polyacrylate, polyacrylamide,polyvinylpyrrolidone, polysaccharide, Zwitterionic polyelectrolyte, acopolymerized system of polymer segments of mixed charges and/or aninterpenetrating blend mixture of cationic and anionic polymers.Preferably, the antifouling reagent is polyethylene glycol. The reactivelinker is thermally reactive and applies to ester-, oxo-, imine-,azole-, methine-, urea-, carbonate-, amide-, carbamate-, disulfide-,siloxane-directed or transition metal-based cross-coupling precursors.Preferably, the reactive linker is maleic anhydride. The basic plasticis a thermoplastic and melt-processable plastic resin includingpolyolefin, polyether, polyvinyl, polyester, polyacetal, polyamide,polyurethane, polyacrylate, polycarbonate, polyimide, polyphthalate,polysulfone, polythioether, polyketone, epoxide and elastomer. Theantimicrobial thermoplastic resin may contain non-biocidal additives,including but not limited to catalysts, initiators, stabilizers, foamingagents, plasticizers, thickeners, lubricants, fillers, impact modifiers,anti-blocks, clarifiers, antistatics, flame retardants and/or colorants.

Referring to FIG. 1, a preparation process of the antimicrobialthermoplastic resin is shown. An intermediate is prepared by grafting areactive linker on a base polymer backbone. A masterbatch resin isprepared by grafting an antifouling reagent onto the intermediate. Themasterbatch resin is melt extruding to prepare a masterbatch and thenthe masterbatch is dried and pelletized after cooling. Melt compoundingthe masterbatch and a basic plastic to prepare the antimicrobialthermoplastic resin. Another preparation route is undergone throughsingle-step extrusion of a dry blend mixture of base plastic andantifouling reagents. Molding into a finished article by a thermoformingprocess. The thermoforming process includes spinning, extrusion,injection, compression, foaming and drawing. The finished article ismolded into a form including solid, monolith, tube, composite, fiber,film, sheet and varnish.

Embodiment 1

In this embodiment, therefore, poly(ethylene glycol) (PEG) was selectedas the antifouling reagent to render the modified base polymers'bacterial repelling property. The masterbatches of PEG derivatives wereintroduced to the commercially available resins: polypropylenes (PP),polyethylenes (PE) and polycarbonates (PC). PP and PE demonstrate lowmelt processing temperatures while PC demonstrates high melt processingtemperature.

1.1 Wet Synthesis of PEG-Bearing Styrene-Maleic Anhydride Copolymer(SMA-PEG) as Masterbatch.

SMA-PEG was prepared by grafting PEG 10,000 (Tianjin Kermel) onto thebackbone of styrene-maleic anhydride (SMA) copolymer (Sigma-Aldrich,Catalog no. 442399) in acetone. In other words, 100 g of PEG 10,000 wasfirst dissolved in 500 mL of boiling acetone to give a 20 wt % PEGsolution. 3.2 g of SMA was subsequently dissolved in the PEG solutionand the reaction mixture was stirred under reflux overnight. Thereaction was quenched in hexane to give a white precipitate. The powderyprecipitate was purified by filtration, followed by vacuum drying atroom temperature. Equation 1 illustrates the synthetic route to obtainSMA-PEG.

From the attenuated total reflection-Fourier transform infrared(ATR-FTIR, Bruker Vertex 70 Hyperion 1000 with PLATINUM ATR, diamondcrystal probe, DLaTGS detector) spectrum of SMA (c.f., dashed line inFIG. 2), the absorption band over 3000 cm⁻¹ corresponds to alkenyl C−Hand C═C stretching modes of unsaturated aromatics, indicating thepresence of styrene units. Another feature is the sharp absorption bandfrom 2000 cm⁻¹ to 1600 cm⁻¹ which is a unique feature of cyclicanhydride, an indication of maleic anhydride in SMA. These two featuresare clearly absent in the ATR-FTIR spectrum of SMA-PEG (c.f., solid linein FIG. 2). The disappearance of the characteristic aromatic band (above3000 cm⁻¹) is mainly due to the decrease of the concentration of styreneunits of SMA-PEG near the surface because ATR-FTIR is asurface-sensitive characterization technique with a sampling depth ofthe order of tens of microns. The attenuation of maleic anhydride bandindicates the complete consumption of maleic anhydride in SMA byesterification with hydroxyl end group of PEG.

1.2 Preparation of PP/SMA-PEG

PP (GD-150, Maoming Petro-Chemical Shihua Company) in powder form wasselected as the base plastic for preparation of antimicrobial PP resins.95 g of PP and 5 g of SMA-PEG were mixed thoroughly in a rotating drummixer (Better Pak International YG-1KG). The drum mixing was performedby repeated 10 clockwise and anticlockwise rotations each for 1 minuteat a speed of 60 rpm. The powdery mixture was subsequently extruded on adesktop single-screw extruder (Wellzoom C-type). The extruder has anozzle diameter of 1.75 mm, length-to-diameter ratio of 10:1 and amaximum screw speed of 10 rpm, where the screw is driven by a 240-Wmotor. In the experiment, the temperature settings were 195° C. at thebarrel and 200° C. at the die of the extruder while the speed of thescrew was 5 rpm for extrusion. The extrudate in the form of shortfilaments was cooled down in air and further cryogenically granulatedinto powders with a swing-type stainless steel three-blade pulverizer(Laifu LFP-2500A).

The thermogravimetric (TGA, TA Q5000, at a heating rate of 5° C./minwith an air flow rate of 25 mL/min) result (c.f., FIG. 3a ) onPP/SMA-PEG sample shows a peak decomposition temperature to be slightlybeyond 250° C. alongside a substantial weight loss in air. Certainextent of reduction of the decomposition temperature as compared withthe base PP plastic (298° C.) was expected. Although PEG is susceptibleto oxidative degradation which worsens upon heating, the TGA result onPP/SMA-PEG has evidently excluded the thermal stability concern with PEGmodification. FIG. 3b shows the differential scanning calorimetry (DSC,TA Q1000) thermograms of PP/SMA-PEG resin in one heating-cooling cycleat a ramping rate of 5° C./min in nitrogen atmosphere. Two endothermicpeaks were observed on the heating trace: a peak at 61° C. correspondingto the melting of PEG and a peak at 168° C. corresponding to the meltingof PP, thus making it suitable for injection molding or other subsequentthermoforming processes for article formation which fits within theprocessing window of 30 to 50° C. above its melting temperature. Theevolution of the two peaks indicates the formation of phase separatedmorphology of the as-extruded PP/SMA-PEG resin.

The melt flow index of PP/SMA-PEG was measured on a melt indexer (ModelKL-MI-BP, Dongguan Kunlun Testing Instrument Company) to be 13.20 g/10min at 190° C. under the load of 2.16 kg, a value higher than that ofthe pristine PP (GD-150) which was 9.68 g/10 min. At 230° C. under thesame load, the melt of PP/SMA-PEG resin flowed like water very rapidly.

1.3 Fabrication of Antimicrobial PC Resins (PC/SMA-PEG) IncorporatingAntifouling Masterbatch

PC/SMA-PEG was prepared by single-screw extrusion of a blend of PC(Teijin Panlite® L1225Y) granules (95 g) and SMA-PEG powders (5 g) whichhave been pre-mixed in a drum mixer by the same condition as in theforegoing sub-section 1.2. The barrel and the die temperatures forextrusion were set at 275° C. and 280° C. respectively. The speed of thescrew was adjusted to 3 rpm due to the low melt viscosity of PC. Theextrudate in the form of short filaments was cooled down in air and thencryogenically granulated into powders with a three-blade pulverizer.

The melt flow index of the PC/SMA-PEG resin was measured to be 21.60g/10 min which was nearly double that of the pristine PC (11.68 g/10min) at 300° C. under the same load of 1.2 kg.

There were two distinct decomposition peak temperatures for thePC/SMA-PEG samples: one near 400° C. and another one near 550° C. (c.f.,FIG. 4a ). The first decomposition peak might correspond to thedecomposition of the resin rich in SMA-PEG content. The firstdecomposition temperature was not reduced as much compared with that ofthe pristine PC which was recorded to be 445° C. Evidence of thepresence of SMA-PEG was located on the DSC heating curve of the sample(c.f., FIG. 4b ), wherein an endothermic peak was shown at 60° C. Thispeak is assigned as the melting temperature of SMA-PEG. Since PC isintrinsically an amorphous polymer, no melting peak should be acquiredfor PC despite the typical second-order endothermic transition at 150°C., which is assigned as the rubber-glass transition temperature of PC.The first-order melting transition observed in PC/SMA-PEG thereforerepresents the SMA-PEG phase domains in the PC matrix. An additionalexotherm is observed at about 327° C. attributing to some extent of bondformation events, most probably transesterification reactions between PCand the free hydroxyl ends of SMA-PEG.

1.4 Fabrication of Antimicrobial PE Resins (PE/SMA-PEG) IncorporatingAntifouling Masterbatch

PE/SMA-PEG was prepared by twin-screw extrusion of a blend of PE (SABIC®1922SF) granules (500 g) and SMA-PEG powders (25 g) which have beenpre-mixed in a drum mixer by the same condition as in the foregoingsub-section 1.2 or 1.3. The extrusion was undergone on a co-rotatingtwin-screw extruder (Model AK26, Nanjing KY Chemical Machinery) with alength-to-diameter ratio of 44:1, a screw diameter of 26 mm and amaximum screw speed of 600 rpm. The screw rotation was driven by a 7.5kW motor. The extruder was connected in line with a water bath followedby a pelletizer. The barrel temperature profile from the front to therear (with a total of 8 temperature zones) was read as: 150° C., 160°C., 170° C., 170° C., 170° C., 170° C., 170° C., and 160° C. The feedfrequency was 2 Hz and the speed of the screw was 150 rpm. The extrudatewas solidified from melt upon cooling in water and finally subjected topelletization. The plastic pellets were then dried in oven at 50° C.overnight.

Embodiment 2

2.1 Wet Synthesis of PEG-Bearing Maleated Polypropylene (PP-MA-PEG) asMasterbatch

PP-MA-PEG was prepared by wet chemistry via grafting of PEG 2000(Tianjin Kermel) onto maleated polypropylene (PP-MA) (Sigma-Aldrich,Catalog no. 427845) with a weight average and number average molecularweight of 9100 and 3900, respectively, under reflux in toluene. 10 g ofPP-MA was first dissolved into 200 mL of boiling toluene to give a 5 wt% solution of PP-MA. 20 g of PEG 2000 was added into the PP-MA solution.The reaction mixture was refluxed overnight and quenched in hexane togive product as a white precipitate. The precipitate was filtered toremove unreacted PEG and finally dried at room temperature in vacuum.The reaction scheme is demonstrated in Equation 2.

FIG. 5 shows the ATR-FTIR spectrum of PP-MA-PEG sample. A strong peakemerged at 1115 cm⁻¹, corresponding to the C—O stretching mode due tothe ether (—O—CH₂—) linkage in PEG. On the other hand, the band typicalof the unreacted maleic anhydride groups that should appear at about1780 and 1856 cm⁻¹ for symmetric and asymmetric >C═O frequency of theC═O groups on the cyclic anhydride structure, respectively disappearedon the ATR spectrum of PP-MA-PEG. The C═O band seen between 1725 cm⁻¹and 1500 cm⁻¹ could represent the hydrolysed maleic anhydride groups. Atleast one hydroxyl end group of PEG was covalently grafted to the PPbackbone by esterification through ring opening of the anhydridestructure and this might result in a partially converted acid form ofthe maleic anhydride group. The result confirms that PEG was engraftedonto PP-MA backbone and a portion of PEG chain tended to migrate to thematerial surface.

2.2 Preparation of PP/PP-MA-PEG

A clear grade PP (Total Petrochemicals Lumicene® MR10MX0) in granularform was selected as the base plastic for preparation of antimicrobialPP resins. PP/PP-MA-PEG was prepared by extruding a dry mixture of thePP granules and PP-MA-PEG powders on a drum mixer by the same condition.In this case, 7.5 g of PP-MA-PEG was blended with 92.5 g PP to give atotal 100 g of the mixture which was subsequently extruded on asingle-screw extruder. The temperature settings were 180° C. and 185° C.for the barrel and the die of the extruder and the speed of the screwwas 7 rpm for extrusion. The extrudate in the form of short filamentswas cooled down in air and then cryogenically granulated into powderswith a three-blade pulverizer.

FIG. 6a shows the TGA weight loss curve of PP/PP-MA-PEG wherein the peakdecomposition temperature approached to 300° C., a value higher thanthat of the base PP plastic (277° C.). FIG. 6b similarly indicates theformation of the phase separated morphology of the as-extrudedPP/PP-MA-PEG resin by exhibiting two endothermic peaks centered at about57 and 137° C. which were respectively characteristic of PEG and PPconstituents.

Embodiment 3

3.1 Single-Step Extrusion of PEG-Modified Maleated Olefin BearingPolypropylene (PP/PP-MA-C/PEG)

PP/PP-MA-C/PEG was prepared by reactively extruding a dry blend mixtureof three solid resin components: (1) PP (Total Petrochemicals Lumicene®MR10MX0), a random olefin copolymer; (2) PP-MA-C (Dow® Amplify™ GR 216),a maleic anhydride-grafted olefin plastomer and (3) PEG 10,000 (TianjinKermel). Prior to extrusion, 50 g of PEG 10,000 powders were pre-mixedtogether with 100 g of PP-MA-C pellets and 900 g of PP granules on thedrum mixer. The dry blend mixture was fed into the twin-screw extruderfrom the front hopper. The barrel temperatures beginning from the frontto the rear were 170° C., 180° C., 180° C., 180° C., 180° C., 180° C.,180° C. and 170° C. The feed frequency was 2 Hz while the speed of thescrew was 150 rpm. The extrudate was cooled down in a water bath forminga solid filament and finally pelletized with a pelletizer. Thepelletized resins were dried in a ventilated oven at 50° C. overnight.

FIG. 7 shows the ATR-FTIR spectrum of PP/PP-MA-C/PEG as compared withPP-MA-C. The melt flow index of PP/PP-MA-C/PEG was also measured to be4.88 and 13.40 g/10 min respectively at 190 and 230° C. under the loadof 2.16 kg, following the industrial standard of ASTM D1238-10. Theresults were higher than the values of 4.14 and 9.36 g/10 min recordedfor the pristine PP (MR10MX0) under the same conditions. The increase ofthe melt flow index of PP after PEG modification is owing to the factthat PEG had a much lower melting point and molecular weight than PP.This result implicates that the PEG molecules tended to partition tomelt surface during extrusion and therefore maintained the chance toreside PEG in the sample surface upon solidification.

Embodiment 4

4.1 Two-Step Fabrication of N-(4-hydroxyphenyl)Maleimide-Modified PEGBearing Polypropylene (PP/HPM-PEG) as Masterbatch

HPM-PEG, as an antifouling masterbatch precursor, was synthesizedaccording to Equations (3)-(5). N-(4-hydroxyphenyl)maleimide (HPM) wasprepared by addition of maleic anhydride (1.67M) and 4-aminophenol(1.67M) in dimethylformamide (DMF) followed by intramolecularcondensation via a highly hygroscopic phosphorus pentoxide reagent(0.33M) and concentrated sulphuric acid (0.42M) leading to a closed ringmaleimide group in a one-pot reaction. Hydroxyl end groups of a PEG10,000 (Tianjin Kermel) chain were subsequently activated by tosylationin a 1:5 molar ratio to tosyl chloride to generate PEG-OTs in thepresence of a base catalyst, such as triethylamine (Et₃N, 0.25M indichloromethane). 0.025 M PEG-OTs and HPM in a 1:1 molar ratio werereacted under reflux in acetone in the presence of excess triethylamine(20:1 with respect to HPM) for at least two days. p-toluenesulfonic acidwhich was one major by-product formed insoluble acid-base adducts withtriethylamine and precipitated out from the reaction solution. Finally,the acetone solution was filtered to remove the adduct by-products andthen subjected to precipitation to yield HPM-PEG as deep orange powdersin diethyl ether in the second step of filtration.

In the second step, prior to the extrusion process, 500 g PP (GD-150,Maoming Petro-Chemical Shihua Company), 2.5 g2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (Arkema Luperox® 101XL45),and 25 g HPM-PEG were pre-mixed on the rotating drum mixer by the samecondition as in the foregoing sub-section 1.2, 1.3 or 1.4. Luperox®101XL45, a heat-sensitive initiator, was added to generate free radicalsby abstraction of hydrogen atoms from PP to allow additive coupling ofPEG to PP via the maleimide group by extrusion. The reactive extrusionof the ternary blend was performed on the twin-screw extruder. Thebarrel temperature profile from the front to the rear was read as: 190°C., 200° C., 200° C., 200° C., 200° C., 190° C., 190° C., and 190° C.The feed frequency was 2 Hz and the speed of the screw was 150 rpm. Theextrudate was solidified from melt upon cooling in water and finallysubjected to pelletization. The plastic pellets were then dried in ovenat 50° C. overnight to remove the absorbed moisture.

The melt flow index of the pristine PP (GD-150) was measured to be 9.68and 43.6 g/10 min at 190° C. and 230° C. under the load of 2.16 kg. Themelt flow index of PP/HPM-PEG resin was however not measureable even at190° C. due to extremely high fluidity under the same load condition.The increase in melt flow index of the resin helps solve the moldfilling problem by increasing the injection speed rather than increasingthe processing temperature. A higher melt flow is recommended forinjection molding in particular for thin-walled applications owing tothe prerequisite of high shear rates being encountered. FIG. 8 is anATR-FTIR spectrum of N-(4-hydroxyphenyl)maleimide-modified PEG bearingpolypropylene (PP/HPM-PEG).

Embodiment 5

5.1 Bacterial Adsorption Studies on Antimicrobial Thermoplastic Resinsand Masterbatches Thereof

Test specimens in circular discs with diameter and thickness of 64 mmand about 1 mm respectively were prepared by melting and fusion of thesolid resin samples in a glass Petri dish on a heating plate.Solidification was done by cooling in atmospheric pressure at roomtemperature. To examine the bacterial adhesion and growth behavior onthese pristine or modified specimen surfaces after thermoforming, swabtest was achieved by collecting the adherent bacteria from the specimensurface using a cotton tip applicator (Medicom) prior to incubation withinoculums for the time elapsed for bioburden challenge test. Originally,plate counting of colonies along with serial dilution (in case ofobtaining large colony populations) is used to determine the amount ofviable bacteria in the inoculums after contact with products for adesignated time at a given temperature. Slight modification on theprotocol was made for executing the bacterial adsorption studiesinvolving swabs. The tests were based on the original culturemethodology but the amount of bacteria being attached to the productsurface after contact was determined to assess the bacteria repellentperformance of the plastic samples and their resistance towardsbacterial colonization. In brief, a test inoculum of a selectedEscherichia coli strain (ATCC® 8739™) was prepared and enumerated uponincubation according to the Japanese industrial standard (JIS Z2801:2000) by finally adjusting the OD₆₀₀ of inoculum to 0.5 determinedwith a microplate reader (Molecular Devices SpectraMax M3). Thiscorresponds to a population of approximately 10⁸ bacteria counts permillimeter of a 1/500 nutrient broth medium. Bioburden challengeprocedure was subsequently carried out by incubating the inoculum ofEscherichia coli (3 ml) over one face of a thermoformed plastic discsample at 37° C. for 24 hours, followed by rinsing with 0.9% w/v salinefor two to three times. The adherent bacteria remaining on the rinsedsample surfaces, which were inclined to biofilm growth, were swabbed andthen dislodged to 1 millimeter of 0.9% w/v saline on a vortexer toperform conventional spread plating.

FIGS. 9 and 10 show the Escherichia coli colonies developed from theadherent species collected from several examples of antimicrobialmasterbatches and resins deriving from PP and PC as the base plasticsrespectively: PP/PP-MA-C/PEG, PP/HPM-PEG, PP/SMA-PEG, PP/PP-MA-PEG andPC/SMA-PEG in comparison with their respective controls (pristine baseplastics) without treatment. The bacterial adsorption studies wererepeated once by using two separate test specimens for each sample. Thecomplete set of spread count data is compiled in Table 1 which indicatesthat PP and PC after modifications with PEG-derived compounds ormasterbatches by different methods demonstrated notable improvement ofresistance towards Escherichia coli adsorption onto the sample surfaces.

Table 1 is a summary of plate count results of Escherichia coliadsorption on thermoformed specimens.

Dilution (Colony Forming Unit) Samples 10x 100x 1000x Resins withPP(MR10MX0) #1 >300 >300 41 low processing PP(MR10MX0) #2 >300 >300 46temperature PP/PP-MA-C/PEG #1 148 15 1 PP/PP-MA-C/PEG #2 >300 65 4PP/PP-MA-PEG #1 15 1 0 PP/PP-MA-PEG #2 0 0 0 PP(GD-150) #1 >300 >300 34PP(GD-150) #2 >300 >300 44 PP/SMA-PEG #1 0 0 0 PP/SMA-PEG #2 0 0 0PP/HPM-PEG #1 0 0 0 PP/HPM-PEG #2 0 0 0 Resins with PC #1 >300 >300 >300high processing PC #2 >300 >300 197 temperature PC/SMA-PEG #1 10 0 0PC/SMA-PEG #2 12 0 0

5.2 Bacterial Adsorption Studies on Specimens Injection-Molded fromAntimicrobial PE Resins (PE/SMA-PEG)

The pristine PE and PE/SMA-PEG resins were injection-molded into plasticcircular dishes with a diameter and height of 50 mm and 15 mmrespectively via a desktop vertical plunger-type injection moldingmachine (Model AB-400M, A.B.Machinery, Canada). The barrel temperaturewas 200° C. The pressure was 60 psi.

The bacterial adsorption studies were performed to directly observe theEscherichia coli attached on the surface of the sample.

10 mL of diluted Escherichia coli suspension in 1/500 nutrient brothmedium with a cell density of approximately 10⁴ cells/mL was added intothe injection-molded plastic dishes injection-molded from PE/SMA-PEG.The dishes were directly incubated against the bacterial suspension at37° C. for 24 hours, followed by rinsing with 0.9% w/v saline for two tothree times. 6 mL of nutrient agar solution at about 50° C. was pouredto the dishes and further incubated overnight at 37° C. The agarsolution gradually solidified as nutrient source and those viablebacteria which had adhered strongly to the dish surface developed intocolonies at the bottom of the plastic dishes. FIG. 11 demonstrates thedevelopment of colonies between PE/SMA-PEG and the pristine PE withouttreatment. Colonies developed only on the dish injection-molded from thepristine PE meaning that the specimens injection-molded from PE/SMA-PEGresins was able to prevent initial inhabitation of bacteria.

Embodiment 6

6.1 Single-Step Extrusion of PEG-Containing Polypropylene in thePresence of Styrene-Maleic Anhydride Copolymer and Citric Acid(PP/SMA/PEG/CA)

A dry blend mixture of the following solid components and additives: (1)PP (Total Petrochemicals Lumicene® MR10MX0), a random olefin copolymer;(2) SMA (Sigma-Aldrich, Catalog no. 442399), a styrene-maleic anhydridecopolymer; (3) PEG 10,000 (Aoki Oil Industrial Co., Ltd.) and (4) CA,anhydrous citric acid (Acros, Organics, Manufacturer part no.423565000). The extrusion was undergone on a co-rotating twin screwextruder (Model TE-35, Jiangsu (Nanjing) Keya Co. China) with a screwdiameter of 35.6 mm, a length-to-diameter ratio of about 45 and amaximum screw speed of 600 rpm as driven by an 11-kW motor. A mixture of200 g of PEG 10,000, 6 g of SMA, 40 g of citric acid and 4 kg of PP(MR10MX0) granules was fed into the extruder from the main hopper. Thebarrel temperatures beginning from the front to the rear (with a totalof 8 temperature zones) were 190° C., 200° C., 210° C., 210° C., 200°C., 200° C., 200° C. and 200° C. The feed frequency was 5 Hz and thescrew speed was 300 rpm. Pelletized resins were dried in a hot air dryerat 70° C. for 3 hours before use.

Table 2 is a summary of the physical properties of PP/SMA/PEG/CA,demonstrating minimal effects of antimicrobial treatment on theproperties of the pristine PP.

Properties PP (MR10MX0) PP/SMA/PEG/CA Tensile strength¹/MPa 30 25Optical transmission²/% 85 80 Volume resistivity³/Ω · cm 7.4 × 10¹¹ 8.0× 10¹² Dielectric strength⁴/kV mm⁻¹ 40 35 Thermal conductivity⁵/W m⁻¹k⁻¹ 0.2 0.2 ¹Per ASTM D638: Type V^(c) specimens, injection-molded,tensile speed 100 mm/min, 23 ± 2° C. ²Per ASTM D1003: thickness 1.3 mm(0.05 in) ³Per ANSI/ESD STM11.12: one face of a hot-pressed filmspray-coated with graphite as electrode ⁴Per ASTM D149-09 (Method A):one face of a hot-pressed film spray-coated with graphite as electrode⁵Measured with a light flash apparatus (NETZSCH LFA 467 HyperFlash ®model) on injection-molded circular discs (diameter 12.7 mm; thickness2.0 mm) with silver coating (100 nm) on flat surfaces by vacuumevaporation

Plastic jelly cups were prepared from PP/SMA/PEG/CA using a 100-toninjection molding machine (Cosmos TTI-model, Welltec IndustrialEquipment Ltd.). The jelly cups passed the chemical migration testsperformed in an accredited agency. The tests comply with nationalregulations, including EU No. 10/2011 (overall migration against both 3%w/v acetic acid and 20% v/v ethanol as food stimulants at 20° C. for 6hours and US FDA 21 CFR 177.1520(c), Items 3.1a and 3.2a as apolypropylene copolymer for intended uses in food contact articles). Thejelly cups also comply with the specific migration limits of the heavymetals (barium, cobalt, copper, iron, lithium, manganese and zinc) andprimary aromatic amines for the specific stimulant used (3% w/v aceticacid) at 20° C. for 6 hours as set out by EU No. 10/2011.

6.2 Bacterial Adsorption and Biocompatibility Studies on Hot-PressedFilm Specimens from Antimicrobial PP Resins (PP/SMA/PEG/CA)

To carry out bacteria adsorption and biocompatibility tests on thehot-pressed film samples of PP/SMA/PEG/CA, a test inoculum of a selectedEscherichia coli strain (ATCC® 8739™), a Gram-negative andStaphylococcus aureus strain (ATCC® 6538™), a Gram-positive, wereprepared and enumerated upon incubation by finally adjusting the OD₆₀₀of inoculum to 0.6 and 1.5 respectively and then diluted by 10 times and500 times using 1/500 nutrient broth medium. These resulted in abacterial population of approximately 1×10⁷ and 2×10⁵ cells/mL.Bioburden challenge was subsequently performed by inoculating eitherEscherichia coli or Staphylococcus aureus (2 mL each) over one face ofthree separate film specimens at 37° C. for 24 hours, followed byrinsing with 0.9% w/v saline for two to three times. The inoculatedsurface was swabbed after 24 hours and then dislodged to 1 mL of 0.9%w/v saline on a vortexer. Plating of the bacterial suspension wasrendered with a spiral plater (Eddy Jet 2, IUL Instruments) coupled toan image analyzer (Flash & Go Colony Counter, IUL Instruments), avoidingmultiple dilution procedures.

Table 3 is a summary of bacterial counts expressed in terms of the unitof colony forming units (CFU) per mL against spiking of highconcentration of Escherichia coli and Staphylococcus aureus.

Bacteria Sample/CFU mL⁻¹ #1 #2 #3 Average Escherichia coli PP (MR10MX0)9.40 × 10³ 1.55 × 10³ 1.11 × 10⁴ 7.35 × 10³ PP/SMA/PEG/CA 1.02 × 10²6.10 × 10¹ 4.07 × 10¹ 6.79 × 10¹ Staphylococcus PP (MR10MX0) 4.64 × 10⁴2.22 × 10² 8.07 × 10¹ 2.56 × 10³ aureus PP/SMA/PEG/CA 6.10 × 10¹ 6.10 ×10¹ 6.10 × 10¹ 6.10 × 10¹

Biocompatibility of the film specimens hot-pressed from antimicrobial PPresins was tested by Cell Counting Kit (CCK)-8 colorimetric assay(Dojindo Molecular Technologies) for in-vitro cytotoxicity assessment inaccordance with the minimum essential medium (MEM) elution methodologydescribed by the ISO 10993-5 standard as described in FIG. 13.

Time/h Test Procedure 00:00 Seed 96-well plates: 5000 cells/100 μL DMEMculture medium per well Incubate (37° C./5% CO₂/24 h) ↓ 24:00 Removeculture medium ↓ 24:00 Treat with the extract of the test samples inDMEM (100 μL) (untreated blank = DMEM) Incubate (37° C./5%CO₂24 h or 48h) ↓ 72:00 Add 10 μL of CCK-8 (Dojindo) solution Incubate (37° C./5%CO₂/4 h) ↓ 52:00 Measure the absorbance at 450 nm (i.e. cell viability)or 76:00

Slight modification of the protocol was done by choosing a differentcell line to ensure that the antimicrobial PP (PP/SMA/PEG/CA) is safe onhuman skin contact. The selected human cell line was HaCaT (CLS CellLines Service GmbH, Germany), an epidermis cell type (keratinocyte).Three films each of the antimicrobial PP and the pristine PP wereprepared for cytotoxicity analyses. The cell viability in the medium wasset to be 100%. FIG. 12 indicates that after 24 and 48 hours ofincubation against the test pieces, the antimicrobial PP wasnon-cytotoxic to HaCaT cells as in the case of the pristine PP showingmore than 90% of cell viability.

6.3 Bacterial Adsorption Studies on Hot-Pressed Film Specimens fromAntimicrobial PVC Resins (PVC/SMA/PEG/CA)

Almost equivalent formulation of PP/SMA/PEG/CA in the foregoingsub-sections 6.1 and 6.2 was exploited for flexible PVC (LG Chem HB-65)as the base plastic for PVC/SMA/PEG/CA. In brief, 100 g of flexible PVCgranules were pre-mixed with 5 g of PEG 600 (acquired from ShanghaiZhanyun Chemical Co., Ltd.), 0.15 g of SMA and 1 g of CA and thenextruded from a single-screw extruder (Wellzoom C-type) at thetemperature settings of 160° C. (barrel) and 165° C. (die) and a screwspeed of about 10 rpm. Pelletized resins were consequently dried at 50°C. overnight before use. One face of the hot-pressed film were similarlychallenged against Staphylococcus aureus at 37° C. for 24 hours as inthe case of PP/SMA/PEG/CA, followed by rinsing with 0.9% w/v saline fortwo to three times. The inoculated surface was swabbed after 24 hoursand then dislodged to 1 mL of 0.9% w/v saline on a vortexer. Countingwas then done with the aid of a spiral plater and an image analyzer.

Table 4 is a summary of adsorbed bacterial counts after bioburdenchallenge against Staphylococcus aureus that the surfaces ofantimicrobial PVC were completely repellent against the bacteria.

Sample/CFU · mL⁻¹ #1 #2 #3 Average PVC (HB-65) 4.33 × 10³ 1.75 × 10⁴1.41 × 10⁵ 5.43 × 10⁴ PVC/SMA/PEG/CA 0 0 0 0

What is claimed is:
 1. A method for preparing an antimicrobialthermoplastic resin, comprising the steps of: preparing an antimicrobialthermoplastic resin by either melt extruding a masterbatch and a basicplastic selected from the group consisting of a polyolefin and apolycarbonate, wherein said antimicrobial thermoplastic is biocide-freeand has bacterial-repellent properties; and molding the antimicrobialthermoplastic resin into a finished article through a thermoformingprocess, wherein said masterbatch comprises a co-polymer comprising apolystyrene repeating unit and a repeating unit represented by:

wherein PEG is polyethylene glycol; a co-polymer comprising apolypropylene repeating unit and a repeating unit represented by:

a polymer comprising a polypropylene repeating unit crosslinked with anagent represented by:

a co-polymer comprising a polypropylene repeating unit and a repeatingunit prepared by the reaction of a maleic anhydride-grafted olefinplastomer and polyethylene; drying the masterbatch; and pelletizing themasterbatch.
 2. The method of claim 1, wherein the thermoforming processis selected from a group consisting of spinning, extrusion, injection,compression, foaming and drawing.
 3. The method of claim 1, wherein thefinished article is in a form selected from a group of solid, monolith,tube, composite, fiber, film, sheet and varnish.
 4. The method of claim1, wherein the basic plastic is a thermoplastic and melt-processableplastic resin selected from a group consisting of polyethylene,polypropylene.
 5. The method of claim 1, wherein the step of preparing amasterbatch by grafting an antifouling reagent onto the intermediate isperformed by a wet chemistry method.
 6. An antimicrobial thermoplasticresin prepared by the method of claim
 1. 7. The method of claim 1,wherein the masterbatch is present at a concentration of: 4.7% to 7.5%(m/m) in the antimicrobial thermoplastic resin.
 8. The method of claim7, wherein the basic plastic is polyethylene, polypropylene, orpolycarbonate.